ChemistryStepByStep

Molality Calculator

Enter moles of solute and kilograms of solvent to instantly get molality (mol/kg).

Molality
4.0000 mol/kg
4.0000 m

m = mol solute / kg solvent = 2 / 0.5 = 4.0000 mol/kg.

What Is Molality?

Molality is moles of solute divided by kilograms of solvent: m = mol / kg. It's easy to confuse with molarity, which divides by liters of the whole solution instead -- molality deliberately ignores the solute's own volume and the final solution's total volume, counting only the solvent's mass.

Because mass doesn't change with temperature the way volume does, molality is the concentration unit of choice whenever temperature itself is part of the calculation, most notably freezing-point depression and boiling-point elevation.

Worked example

What is the molality of a solution made by dissolving 90 g of glucose (C6H12O6) in 0.5 kg of water?

Glucose's molar mass is 180.156 g/mol, so 90 g is 0.4996 mol.

m = mol / kg = 0.4996 / 0.5 = 0.9991 mol/kg.

Common mistake

Using the total solution's mass instead of just the solvent's mass. If you weighed the whole mixed solution, subtract the solute's mass first -- molality's denominator is solvent only.

Keep going

  • Molality and molarity are easy to mix up; the molarity calculator uses solution volume as the denominator instead of solvent mass, and the two only match closely in dilute aqueous solutions. Molarity Calculator
  • Molality needs moles of solute as an input; the mole calculator converts a solute's mass into moles first using its molar mass. Moles Calculator

Frequently Asked Questions

What is molality?

Molality (m) is moles of solute per kilogram of solvent, written mol/kg. It's a different denominator than molarity, which uses liters of the whole solution -- molality only counts the solvent's mass, not the solute or the final solution's volume.

Why use molality instead of molarity?

Molality doesn't change with temperature, since mass doesn't expand or contract when heated the way volume does. That makes it the right choice for colligative-property calculations (freezing-point depression, boiling-point elevation) where temperature itself is what's changing.

Is molality the same as molarity for dilute water solutions?

Close, but not identical. For a dilute aqueous solution at room temperature, 1 kg of water occupies almost exactly 1 L, so molarity and molality are numerically similar -- but they diverge as concentration increases or the solvent isn't water.

How do I get kilograms of solvent if I only measured the total solution mass?

Subtract the solute's mass from the total solution mass to get the solvent's mass alone, then convert to kilograms. Molality only ever counts the solvent, never the solute or the combined solution.

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